1. Field of the Invention
The present invention relates to back fire helical antennas for use in a navigation system such as GPS.
2. Description of the Background Art
With recent development of an information society, a mobil radio communication and a satellite communication have been flourished, and a navigation system such as GPS for receiving radio waves of artificial satellites and detecting the position and speed of mobile bodies is put into practice. In GPS, a radio wave with a frequency of an L band is used, and a back fire helical antenna, spiral antenna and the like are used in practice as a receiving antenna.
FIG. 11 is a perspective view of a conventional back fire helical antenna. A flexible substrate film 3 is lapped around outer peripheries of a cylindrical bobbin 1 being a dielectric. Bobbin 1 serves to retain flexible substrate film 3 in a cylindrical form. Four helical radiation conductors 5, 7, 9 and 11 are formed on a surface of flexible substrate film 3 by etching.
A coaxial cable 38 is disposed at the position of a central axis of cylindrical bobbin 1. Coaxial cable 38 includes a coaxial central conductor 39, an insulator 41 provided around coaxial central conductor 39, and a coaxial outer conductor 43 provided around insulator 41.
An arm 13 is soldered by solder 45 to a first end of coaxial central conductor 39. A first end 23 of radiation conductor 5 is soldered by solder 19 onto a first end 15 of arm 13. A first end 25 of radiation conductor 7 is soldered by solder (not shown) onto a second end 17 of arm 13.
An arm 27 is soldered by solder 47 to a first end of coaxial outer conductor 43. A first end 35 of radiation conductor 9 is soldered by solder 33 to a first end 29 of arm 27. A first end 37 of radiation conductor 11 is soldered by solder (not shown) onto a second end 31 of arm 27.
A lower connection piece 49 is soldered to coaxial outer conductor 43 by solder. Respective second ends 59, 61, 63 and 65 of respective radiation conductors 11, 7, 5 and 9 are soldered, respectively, to first, second, third, and fourth connecting portions 51, 53, 55 and 57 of lower connection piece 49. Reference numerals 67 and 69 denote solders. Radiation conductors 5, 7, 9 and 11 are formed to wrap around bobbin 1.
A helical antenna operation shown in FIG. 11 will now be described. An overall length of a first loop comprised of radiation conductors 5 and 11, arms 13 and 27 and lower connection piece 49 is set to be slightly longer than a wavelength for use, and an overall length of a second loop comprised of radiation conductors 7 and 9, arms 13 and 27 and lower connection piece 49 is set to be slightly shorter than a wavelength for use. At the wavelength for use, the first longer loop exhibits inductive impedance, while the second shorter loop exhibits capacitive impedance.
Thus, provision of a suitable difference in the overall lengths of both loops results in a mutual phase difference of 90.degree. between respective currents flowing through mutually adjacent radiation conductors 5, 7, 9 and 11 despite the fact that both loops are fed with power in parallel, so that a circularly polarized wave is efficiently radiated.
FIG. 12 is a sectional view of coaxial central conductor 39, and FIG. 13 is a sectional view of insulator 41. Coaxial central conductor 39 has such a structure that the conductor has a different diameter only by the length of .lambda..sub.g /4 from a feeder 75 which is a connection portion with arm 13.
This part is called a coaxial central conductor 39a. Coaxial central conductor 39a, insulator 41 and coaxial outer conductor 43 constitute a impedance transformer. The impedance transformer serves to take a match between impedance of coaxial cable 38 and impedances of radiation conductors 5, 7, 9 and 11. .lambda..sub.g is a wavelength of a radio wave for use. While the diameter of coaxial cable 38 is made larger by the length of .lambda..sub.g /4 in this example, this value varies depending on the impedance of coaxial cable 38 and the impedances of radiation conductors 5, 7, 9 and 11.
As shown in FIG. 13, a cavity 73 of insulator 41 is processed so that coaxial central conductor 39a fits in the cavity.
However, it is difficult to process coaxial central conductor 39 and cavity 73 in the forms shown in FIGS. 12 and 13, leading to a poor productivity of the back fire helical antenna.
In addition, in a conventional quadrifilar back fire helical antenna, since radiation conductors 5, 7, 9 and 11, arms 13 and 27 and lower connection piece 49 are separate parts, the number of places for soldering increases at the time of assembly, and also the number of working steps increases.
As a method for providing a difference in overall lengths of loops, a method for changing a pitch angle of the loops is known as disclosed in Japanese Patent Laying-Open No. 63-26004. A technique in which a parasitic object is disposed in the vicinity of a driver element and phases of currents flowing through radiation conductors 5, 7, 9 and 11 can be changed is disclosed in Japanese Patent Laying-Open No. 2-127804.
In such conventional techniques, however, a structure for realizing a desired loop length is complicated. Further, a structure for controlling phase of a current is complicated. In some case, it is difficult to assemble an antenna and also to control phase of a current flowing through a radiation conductor after completion of the assembly of the antenna.